Understanding "TR" and "OP" Durability Ratings

The TR and OP durability ratings published by the Architectural Woodworking Institute refer to specific categories of finish, and have to be understood to be useful. August 22, 2013

QuestionI have been using ML Campbell products for a number of years but my supplier has no expertise with the product. I have a supplier of Chemcraft products available and I am considering going with them. I have been using Magnamax and MagnaClear precat and while I love the lack of mixing and cleaning with the precats, I am interested in the Chemlife 24 product because of the TR 6 rating but I have no idea of the difference between the TR 4 rating of Magnamax and the TR 6 rating of Chemlife 24. Could someone point me to a list of TR rating explanations?

From the original questioner:Thanks - that clears up a misunderstanding I had about TR ratings. But it does mean I will need to do my own testing to determine if the finish is durable enough for my purposes.
From contributor F:Since you are already with MLC, you should try Klearvar. It has recently been approved for stair treads and flooring with a TR-6 rating.
From the original questioner:The MLC rep has been in touch and I may be able to find a way to keep using their product (better support options). I will give the Klearvar a look.

I took a look at the link to the article in the Knowledge Base and I would not recommend it as a reference on this subject.

Once in a while you’ll see posts where people refer to the TR/OP ratings. And some coatings manufacturers use them in their marketing literature. Sometimes the information is misleading or wrong. Knowing what they really mean won’t make you a better finisher, but it will help you make informed decisions about the coatings you choose and why they are the best choice for the application you have in mind.

Where do the TR/OP ratings come from?

The Architectural Woodwork Institute (AWI) publishes a book called the Architectural Woodwork Quality Standards. As a point of interest, one of our forum participants, Ron Bryze, contributed to a recent version of this document (kudos!). In earlier versions, Section 1500 “Factory Finishing”, contained a chart that compared the performance characteristics of various topcoats that used the “TR & OP” code rating system. Some of the information is dated, but overall it’s still a handy reference

What do the ratings mean?

The various types of topcoats are listed left to right at the top of the chart on page 449. Each category is assigned a Transparent System Code (TR) number (e.g., TR-1) and an Opaque System Code (OP) number (e.g., OP-8). For the most part, they are organized from least durable on the left to most durable on the right, though CAB & Water Acrylic Lacquer (TR-3) are out of sequence.

The real key to remember about these categories is that each one is reserved for a specific type of coating:
TR-1 is standard nitrocellulose lacquer (aka “NC lacquer” or “NC”) (shellac would come closest to this category)
TR-2 is pre- and post-catalyzed lacquer (aka “pre-cat” and “post-cat”)
TR-4 is conversion varnish (aka “CV”)
TR-6 is catalyzed polyurethane (aka “2K PU” – 2 component polyurethane)
TR-7 is polyester
TR-8 is polyester with a 2K PU topcoat (allows for easier buffing)

How to use the chart

If you look down the left side of the chart you will see a number of performance factors listed with a rating of 1 = Excellent to 5 = Poor. Take a look at the factors and you’ll see these categories:
General durability
Finish clarity
Finish surface flexibility
Stain resistance
Heat resistance
Moisture resistance
Solvent resistance

How the finish performs in all of these categories combined provides the best understanding of its applications, though general durability is a good indicator.

When you choose a finish, first take into account how and where the item will be used. For example, a commercial table top will be exposed to above average use and wear. The cleaning products will be more harsh than what you might typically use in your home. Also, there will be some really hot plates and a lot more spills of all kinds of food, drink (including alcohol), and condiments (e.g., mustard). When choosing a finish that can stand up to these conditions, look for one that has high ratings in these areas. If you also need full fill on ring-porous woods or a high build finish seen in high end yachts, jets, and cars, then a TR-7 or TR-8 would be the best choice.

In contrast to the commercial use application, you may be building and/or finishing a very nice heirloom grade piece of furniture that will be passed down through the generations. In this case, you would be wise to choose NC lacquer or shellac to make refinishing easier in the future. Or… If it’s an item that does not need a film forming finish for protection, and a “close to the wood” appearance is what you’re looking for, a drying oil (e.g., linseed, tung, etc.) or an oil/vanish blend (TR-0) is a good choice.

*** NOTE *** The ratings provided for each category may not be up to date for the coating(s) you’re using. For example, catalyzed poly is given a rating of 4 for finish clarity but 2K acrylic polyurethanes have great clarity. The “general durability” rating is a good place to start when making the initial selection of the most appropriate finish for your application.

Interpret the marketing literature

You may have seen something like this in the marketing literature for the finishes you use or are considering - “This pre-catalyzed lacquer meets the AWI performance standard TR-4 for chemical and moisture resistance.” You know from looking at the AWI chart, and the list provided above, that TR-4 is reserved for conversion varnish and it has an overall durability of 2 while catalyzed lacquer has a 3.

So what is the manufacturer telling us? Read the words closely and you’ll see they define the exact performance factors that can be considered as durable as CV; chemical and moisture resistance (only). If you are doing kitchen and/or bath cabinetry, those 2 performance characteristics are important and you may decide the other properties of CV that make it more durable than catalyzed lacquer are not critical.

Final Note

Properly formulated CV is tougher than catalyzed lacquer and catalyzed poly is tougher than CV. BUT…. different brands have different formulations which gives them different performance properties, both good and bad. If you’re not getting the performance you want from your finish, it may be a simple matter of trying a different brand.

From the original questioner:Thanks for posting that - it does help me sort this out. It seems I was misreading TR rating as a spec that a particular finish had passed a set of tests to indicate a certain durability. I like the MLC sell sheets listing of chemical resistance and adhesion, cold check, edge soak, wet heat resistance etc. but unless you have those same specs for the other manufacturers, it does not help in doing comparisons.

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